1. Field of the Invention
The present invention relates to an information processing system, an information processing method, and a program. More particularly, the present invention is concerned with an information system that verifies a content recorded in an information recording medium, an information processing method, and a program.
2. Description of the Related Art
Disks including a digital versatile disc (DVD) and a Blu-ray Disc (registered trademark) are used as content recording media. For example, a movie content or the like is recorded in disks and provided for users. The disk-recorded content is often a content whose creator or vendor holds a copyright or a distribution right thereon. Various techniques are applied in order to prevent fraudulent use of the content.
As one of techniques for barring fraudulent use of contents, a technique of verifying whether a disk-recorded content has been altered is implemented in an information processing (reproducing) system that executes content reproduction. The information processing system is configured to verify whether a content recorded in a disk has been altered, and extend control so that: only when it is ascertained that the content has not been altered, reproduction of the content will be permitted; and if it is ascertained that the content has been altered, the reproduction of the content will not be executed.
For example, in patent document 1 (JP-A-2007-79989), a configuration in which a hash value is calculated from a content file scheduled to be reproduced, and then compared with a collation hash value recorded in advance in a disk, that is, a collation hash value that has already been calculated based on an authentic content is described.
The configuration is a content usage control configuration in which if the hash values newly calculated based on component data items of the content recorded in the disk square with the collation hash values, a decision is made that the content has not been altered, and reproduction processing to be executed for the content is permitted.
In an advanced access content system (AACS) known as a content copyright management system, a process to be employed in a case where hash values are used to verify a disk-recorded content is stipulated.
In a disk-recorded content conformable to the AACS standards, numerous hash units (HUs) formed with construction data items of the content are defined. For example, several hundreds of hash units to several thousands of hash units are defined in one content. In a disk, verification values associated with the hash units, that is, hash values that have already been calculated from an authentic content are stored as collation hash values.
An information processing system that performs content verification randomly selects several hash units from among numerous hash units, reads component data items (hash units) of the content associated with the selected hash units, calculates hash values from the read hash units, and collates the hash values with the verification values recorded in a disk.
If all of the selected hash units square with the verification values, the disk-recorded content is recognized as an authentic content that has not been altered. The process then proceeds to content reproduction processing. If part of the selected hash units does not square with a counterpart of the verification values, the disk-recorded content is recognized as a content that may be altered. The content reproduction processing is therefore not permitted.
As mentioned above, in content verification processing conformable to the AACS standards, since it takes much time to verify all of several hundreds of hash units to several thousands of hash units with respect to one content, several (for example, seven) hash units are randomly selected in order to perform verification processing.
However, even when several (for example, seven) selected hash units are verified, since the hash units are randomly selected, the selected hash units dispersedly exist at various positions in a disk. Therefore, it takes much time to read the data items of the hash units. This poses a problem in that a standby time which elapses until a content is reproduced gets longer.
For example, assume that the number of hash units to be randomly selected is seven and data items forming the hash units are read in the order in which the hash units are selected. In this case, a head is moved to recording areas of the hash units in the order in which the hash units are selected, that is, so-called seek is executed in order to read the data items. Therefore, depending on the data recording positions of the randomly selected hash units, the seek movement of the head may be a repetition of a movement from the internal circumference of the disk to the external circumference thereof. In this case, a head moving distance increases and data reading processing requires much time.
Further, if a disk is a two-layer disk or has a multilayer structure, the data reading processing requires much more time. As shown in, for example, (a) in FIG. 1, assume that a content stored in the two-layer disk is verified. One content such as a movie is recorded in two layers (an upper layer (layer 0) and a lower layer (layer 1)).
Even when the content recorded in the two-layer disk is verified, hash units that are objects of verification are randomly selected. For example, seven hash units shall be selected as objects of verification. Through random hash unit selection processing, seven hash units (HU1 to HU7) shall be, as shown in (b) in FIG. 1, selected from the upper layer (layer 0) and lower layer (layer 1).
The seven selected hash units (HU1 to HU7) shall be stored at recording positions shown in (b) in FIG. 1 in such a manner that the hash units (HU1, HU3, HU5, and HU7) are disposed in the upper layer (layer 0) and the hash units (HU2, HU4, and HU6) are disposed in the lower layer (layer 1).
Data reading is performed in the order in which the hash units (HU1 to HU7) are randomly selected. First, the hash unit (HU1) is read from the upper layer. Secondly, the hash unit (HU2) is read from the lower layer. Thirdly, the hash unit (HU3) is read from the upper layer. Fourthly, the hash unit (HU4) is read from the lower layer. Thus, recorded data items are read alternately from the upper layer and lower layer. This processing requires not only the seek movement of a head but also refocusing control accompanying layer switching. In this example, the seek and refocusing control have to be repeated multiple times. Eventually, a content verification processing time gets longer.
The example shown in (a) and (b) in FIG. 1 is an example of processing involving a two-layer disk. Even when a single-layer disk having one layer is employed, hash unit reading during which a head has to be reciprocated between the external side of the disk and the internal side thereof is required, though it depends on the results of hash-unit selection. Eventually, a content verification time is extended. Due to the extension of the verification time, beginning of reproduction of a content is delayed. This poses a problem in that a user has to stand by for a long period of time.